 Among his other anatomical contributions was the coining of the term nucleus accumbens, which he described in the brain of the common ringtail possum as part of his survey of the neuroanatomy of the marsupials and monotremes.In 1898 he published "Psychophysiologische Erkenntnistheorie" (Psychophysiological Theory of Knowledge), with psychology being the basis of his philosophic belief system.He was a practitioner of associative psychology, and from a philosophic standpoint advocated monistic positivism, or what he called the "principle of immanence".